Fix _strptime argument checks

ext/date/date_core.c

@@ -27,7 +27,7 @@ static VALUE eDateError;
 static VALUE half_days_in_day, day_in_nanoseconds;
 static double positive_inf, negative_inf;
 
-// used by deconstruct_keys
+/* used by deconstruct_keys */
 static VALUE sym_year, sym_month, sym_day, sym_yday, sym_wday;
 static VALUE sym_hour, sym_min, sym_sec, sym_sec_fraction, sym_zone;
 
@@ -4528,6 +4528,7 @@ date_s__strptime_internal(int argc, VALUE *argv, VALUE klass,
     rb_scan_args(argc, argv, "11", &vstr, &vfmt);
 
     StringValue(vstr);
+    if (argc > 1) StringValue(vfmt);
     if (!rb_enc_str_asciicompat_p(vstr))
 	rb_raise(rb_eArgError,
 		 "string should have ASCII compatible encoding");
@@ -4538,7 +4539,6 @@ date_s__strptime_internal(int argc, VALUE *argv, VALUE klass,
 	flen = strlen(default_fmt);
     }
     else {
-	StringValue(vfmt);
 	if (!rb_enc_str_asciicompat_p(vfmt))
 	    rb_raise(rb_eArgError,
 		     "format should have ASCII compatible encoding");

ext/date/date_strptime.c

@@ -661,16 +661,16 @@ date__strptime(const char *str, size_t slen,
 
     si = date__strptime_internal(str, slen, fmt, flen, hash);
 
+    if (fail_p())
+	return Qnil;
+
     if (slen > si) {
 	VALUE s;
 
 	s = rb_usascii_str_new(&str[si], slen - si);
 	set_hash("leftover", s);
     }
 
-    if (fail_p())
-	return Qnil;
-
     cent = del_hash("_cent");
     if (!NIL_P(cent)) {
 	VALUE year;

test/date/test_date_strptime.rb

@@ -517,7 +517,20 @@ def test_sz
     d = DateTime.strptime('9000 +0200', '%Q %z')
     assert_equal([1970, 1, 1, 2, 0, 9], [d.year, d.mon, d.mday, d.hour, d.min, d.sec])
     assert_equal(Rational(2, 24), d.offset)
-
   end
 
+  def test_format_modified
+    str = " " * 100
+    fmt = Struct.new(:str) {
+      def to_str
+        str << "2026-06-01" << " "*100
+        " %F "
+      end
+    }.new(str)
+    d = Date._strptime(str, fmt)
+    assert_not_nil(d)
+    assert_equal(2026, d[:year])
+    assert_equal(6, d[:mon])
+    assert_equal(1, d[:mday])
+  end
 end